Environmental and economic concerns associated with depleting oil resources have triggered a growing interest in the chemical conversion of carbon dioxide (CO2), so as to enable its use as a renewable carbon source. CO2 is, despite its low reactivity, a highly attractive carbon feedstock, as it is inexpensive, virtually non-toxic, abundantly available in high purity and non-hazardous. Therefore, CO2 could be a promising substitute for substances such as carbon monoxide, phosgene or other petrochemical feedstocks in many processes. One of the developing applications of CO2 is copolymerization with epoxides to yield aliphatic polycarbonates. The development of effective catalysts to make such a process profitable is the subject of continuous research.
In WO2009/130470, the contents of which are incorporated herein by reference in their entirety, the copolymerisation of an epoxide with CO2 using a catalyst of a class represented by formula (I) was described:

WO2013/034750, the contents of which are incorporated herein by reference in their entirety, discloses the copolymerisation of an epoxide with CO2 in the presence of a chain transfer agent using a catalyst of a class represented by formula (II):

Various compounds according to formulae (I) and (II) above were tested for their ability to catalyse the reaction between different epoxides and carbon dioxide. In both WO2009/130470 and WO2013/034750, M is specified as being selected from Zn(II), Cr(II), Co(II), Mn(II), Mg(II), Fe(II), Ti(II), Cr(III)-X, Co(III)-X, Mn(III)-X, Fe(III)-X, Ca(II), Ge(II), Al(III)-X, Ti(III)-X, V(III)-X, Ge(IV)-(X)2 or Ti(IV)-(X)2.
Among the epoxides employed in the copolymerization reactions of the prior art, cyclohexene oxide (CHO) received special interest, as the product, poly(cyclohexene carbonate) (PCHC) shows a high glass transition temperature and reasonable tensile strength. Ethylene oxide, propylene oxide and butylene oxide have also received interest as they produce polymers (polyalkylene carbonates, such as PPC) with elastomeric properties which are useful in many applications e.g. films.
WO2012/037282 discloses a catalyst of formula:

WO2012/037282 indicates that these compounds may be useful for the copolymerization of an epoxide with CO2. WO2012/037282 states that M1 and M2 can be any metal atom. However, these complexes were not tested to determine which if any possessed the necessary catalytic activity.
The inventors have now surprisingly found that bimetallic catalysts having at least one nickel metal centre, are active as polymerisation catalysts. In particular, the inventors have found that bimetallic catalysts having at least one nickel metal centre, and preferably having two nickel metal centres, are better in terms of activity and/or selectivity than the catalysts previously disclosed in the art. In particular, catalysts of the invention have improved activity in relation to di-substituted meso-epoxides (e.g. cyclohexene oxide) and mono-substituted epoxides (e.g. propylene oxide), and furthermore improved selectivity to mono-substituted epoxides.